This invention is in the field of nonlinear amplifiers, and relates more particularly to D.C. current compensation circuits for such nonlinear amplifiers.
Typically, nonlinear amplifiers, such as common-emitter gain stages used in RF or microwave amplifiers, have a nonlinear relationship between D.C. input current and D.C. output current, thus making it difficult to predict what the D.C. output current will be for a given D.C. input current. Furthermore, additional nonlinearities are created by process variations and temperature variations. The inability to precisely predict D.C. output current in conventional nonlinear gain stages is a problem because the D.C. output bias current of the nonlinear amplifier directly influences such parameters as voltage swing, transconductance, and, most importantly, voltage gain. Thus, if the relationship between D.C. input and D.C. output current (e.g. base current and collector current) is highly nonlinear, or highly variable due to temperature and/or process tolerances, then biasing a nonlinear amplifier stage such as a common-emitter stage using a conventional, fixed bias source will result in highly variable and unpredictable performance.
Various prior-art circuits for biasing and temperature compensating nonlinear amplifier stages are known, as shown, for example, in European Patent No. 0 488 443 and U.S. Pat. No. 5,150,076. However, the solutions presented in those references do not provide a simple and effective solution the problems discussed above.
Accordingly, it would be desirable to have a D.C. current compensation circuit for a nonlinear amplifier which is capable of controlling output bias current in a predictable manner, and which is capable of easily setting the output bias current to a desired value to achieve a particular gain and/or compensate for temperature or process variations.